Research On Resource Utilization And Reduction Disposal Process And Mechanism Of Zinc-bearing Gypsum Residue

A massive amount of heavy metals-bearing gypsum residue（HMG）is produced every year in the non-ferrous metallurgical industry,and the stockpiled amount of HMG over the years has reached nearly 10million tons.HMG contains considerable amounts of heavy metals,calcium and sulfur.Long-term storage not only causes the waste of resources but also poses a huge risk to the environment.The traditional disposal methods face challenges of high energy consumption,high carbon emission and serious pollution.Based on the characteristics of lower formation temperature and easy hydrolysis of Ca S,a novel process characterized by“selective reduction roasting-hydrolysis leaching-carbonization”was developed in this study to recover zinc and sulfur and to prepare high-quality calcium carbonate from zinc bearing gypsum residue（ZGR）.In this way,the low-carbon resource utilization and reduction disposal of ZGR can be realized.This doctoral thesis establishes a technical prototype for resource utilization and reduction disposal of ZGR through a systematic and comprehensive study on the selective reduction behaviors and mechanisms of ZGR,the thermodynamics and kinetics of selective leaching of roasted products,carbonization behaviors of leachate,and the control of Ca CO₃ morphology and particle size and other key basic issues.The main conclusions obtained in this thesis are as follows:（1）Through the thermodynamic studies on the selective reduction,the main reactions involved in the reduction of ZGR was determined,the feasibility of selective reduction of Ca SO₄ to Ca S,Zn O to Zn（g）,and Fe₂O₃to Fe was demonstrated,the effects of various factors on the reduction behaviors of the valuable components in residue were clarified,and the predominance-area of the selective reduction and the ranges of process parameters were established,which provide the scientific evidences for the selective reduction.The increase in carbon amount and reduction temperature is not only beneficial to the selective reduction of valuable components,but also avoid the formation of liquid phase in the reaction process.（2）The main reaction courses of gypsum with Zn O and Fe₂O₃ during carbothermic reduction were investigated to clarify the mechanism of the selective reduction.At a temperature below 800℃,Ca SO₄ reacts with Zn O and Fe₂O₃ in the reducing atmosphere to form Ca CO₃,Zn S and Fe S,meanwhile,the unreacted Zn O combines with Ca S to form Zn Ca OS;and then.With the temperature increasing from 800℃ to 900℃,the generated Ca CO₃ decomposes into Ca O,and further reacts with Zn S and Fe S to form Zn Ca OS and Fe₄O₄Ca₃S₃ as intermediate products.In this temperature range,Fe₄O₄Ca₃S₃ would decompose into Fe O and Fe OCa S with higher thermal stability.When the reduction temperature increased above 900℃,Zn Ca OS,Fe OCa S and Fe O further decomposes into Ca S,Zn（g）and Fe under the presence of carbon,and finally achieve the selective reduction of gypsum,Zn O and Fe₂O₃.（3）The results of the selective carbothermic reduction roasting of ZGR indicate that temperature,carbon ratio and time have significant effects on the reduction of calcium sulfate,zinc oxide and iron oxide.The optimum reduction conditions were determined as:temperature of 1000℃,carbon dosage of 25%and time of 60 min.Under these conditions,the formation rate of calcium sulfide and the metallization rate of iron reached95.91%and 84.12%,respectively,and 94.79%of the zinc in the ZGR was reduced and volatilized to the flue dust and concentrated,realizing the selective reduction of ZGR and the separation and recovery of zinc.The main components of the reduction product are Ca S and Fe,and the average particle size is 46.16μm.（4）The leaching thermodynamics of the reduced product show that decreasing the p H,potential and temperature,and increasing the liquid-to-solid ratio,gas-to-solid ratio and gas-to-liquid ratios can promote the leaching of calcium sulfide into the solution as Ca（HS）₂,while the impurities in the calcine remain in the leaching residue in the form of insoluble metal sulfides.In the H₂S-H₂O system,the leaching rates of calcium and sulfur in the calcine reached 89.85%and 95.93%,respectively;the mass of leaching residue was only 18%of the mass of original ZGR,and the concentrations of heavy metal ions in the leachate were all below15 mg/L,which achieved the reduction of ZGR and the efficient and selective hydrolysis leaching of calcium sulfide.The kinetics study revealed that the leaching process of the roasted product conformed to the Avrami model and was controlled by the mixture of interfacial chemical reaction and diffusion.（5）Taking the hydrolysate of the reduction roasted ZGR as the calcium source,the effects of carbonization process parameters on the carbonization behaviors of Ca²⁺and S^2-,the particle size and morphology of Ca CO₃ products were systematically investigated.The results showed that the separation of Ca²⁺and S^2-in the hydrolysate can be realized by controlling the final p H of the solution below 6.5.The decrease of the particle size of carbonized products and the formation of spherical Ca CO₃can be promoted by increasing the initial Ca²⁺concentration,stirring speed,CO₂ concentration and flow rate,decreasing the temperature,and adding Al Cl₃ as a crystal controlling agent.During the carbonization process,the added Al Cl₃ reacted with HS^-in the hydrolysate to generate Al（OH）₃ gel,which adhere to the surface of Ca CO₃ crystal nucleus,thereby inhibiting the growth of calcium carbonate crystal nucleus and inducing its directional crystallization into spherical particles.After the carbonization of hydrolysate,a spherical micro-powder Ca CO₃ product with an average particle size of 2.74μm,a purity of 98.2%,and a whiteness of 93.3%was prepared.The average concentration of H₂S in the exit gas reached 27.82%,which enables producing sulfur using the exit gas.As thus,provided support for the resource utilization of calcium sulfate in ZGR.This thesis includes 82 figures,31 tables and 222 references.